Various solutions are obtained from industrial processes where the waste solutions contain dissolved metal salts. For example, pickling acids, such as hydrochloric and nitric acids, are commonly used in the pickling treatment of iron, stainless steel and other metals to remove scale and impurities. The resulting spent acid is often recycled and regenerated for reuse in various processes to minimize waste.
Various processes are known to recover waste acids and to extract metal oxides from aqueous solutions containing dissolved metal compounds. In one process, the waste solution is treated in a spray roasting process where the solution is fed or sprayed into a reactor and heated. The solution is sprayed into the reactor where the water and acid vapors are driven off to leave the metal salts in the reactor. The metal salts are generally carried to a secondary heating apparatus where the metal compounds are heated to convert the metal compounds to a temperature to decompose and convert the metal compounds into metal oxides. The exhaust gases from the reactor are purified by various processes. For example, the exhaust gases can be treated in a cyclone separator, cooled in a venturi washing device, and then passed through a scrubbing column.
The oxides of the prior processes are generally discharged from the bottom of the reactor by a suitable discharge device such as a rotary vane. The prior spray roasting processes provide a brief retention time in the reactor so that a certain amount of residual acid cannot be removed adequately from the metal oxide particles. As a result, the residual acid remains as an impurity in the metal oxide. When the reactor temperature is raised to completely roast the larger droplets and the droplets on the inside of the spray cone, overheating of the reactor occurs. Overheating of the reactor reduces the quality of the resulting oxide for commercial uses. In particular, the high reaction temperatures reduce the specific surface area of the resulting metal oxide particles to typically about 2.7 m2/g. The low surface area of the metal oxides are usually obtained from the production of iron oxide from solutions having a high iron concentration.
Various devices and processes have been proposed to produce metal oxides of high quality. One example is disclosed in European Patent Application 0895962 which discloses the separate secondary roasting of the oxides after spray roasting. The process is primarily concerned with roasting iron containing solutions for producing iron oxide. The disclosed process is a spray roasting process with a short retention time at a relatively low temperature of approximately 500-600° C. The spray roasting process is followed by a separate secondary roasting with a much longer retention time and a lower temperature of approximately 400-500° C. Under some conditions, the low temperature secondary roasting could produce oxides having a high specific surface area. However, the long retention time for the secondary roasting limits the throughput of the plant to a relatively low level. Moreover, the long retention time of the secondary roasting step requires an expensive and complicated operating plant.
Accordingly, a continuing need exists in the industry for an improved process for treating solutions containing dissolved metal salts.